In recent years, LEDs or light-emitting semiconductor elements in general have been used increasingly in lighting technology because the powers attainable with light sources of this kind are now sufficiently high for relatively large lighting applications. By comparison with other types of light source, LEDs offer the advantage of a relatively greater efficiency. Furthermore, the power and therefore the brightness of an LED can be adjusted in a relatively simple manner, so that light can be generated in almost any required colour shade through a corresponding mixing of different colours.
However, one problem of LED illuminants of this kind, which provide LEDs of different colours, is that these illuminants are not stable with regard to colour-location without an appropriate control or regulation on the basis of thermal influences or ageing factors. Furthermore, since deviations in the light emission of individual LEDs can occur as a result of manufacture, this means that cost-intensive measures must be adopted in order to adjust the power of the individual LEDs. Otherwise, the risk would arise, that ultimately, the colour shade of the mixed light emitted by the illuminant may not correspond to the user's requirements.
In this context, it is known from the prior art, that light emitted from the different light sources can be detected using a sensor, and a regulation adapted to the test result can be implemented during the control of the LEDs. Since information about the light of each individual light source must be available in order to achieve an accurate colour control, steps are taken especially in order to obtain this information. For example, in the case of a method described in EP 056 993 B1 for operating light sources, a calibration is implemented during operation. In this context, the light sources are controlled temporarily during a calibration phase in such a manner that only an individual light source is active in each case within a sequence of individual measuring periods. The brightness information for the currently active light source can therefore be obtained in each case via the sensor, wherein a corresponding control of the LEDs as a whole is then implemented on the basis of this information. In this context, the individual measuring periods are preferably dimensioned to be so short that the regular operation of the illuminant is influenced as little as possible.
The method described in EP 1 056 993 B1 is used widely for the colour control of LED illuminants, however, it has proved problematic that the different LEDs are activated in each case individually during the calibration phases. The consequence of this is that, in spite of the relatively short measuring periods, in which light from an individual LED or respectively of an individual colour is emitted, slight colour changes in the light emission of the illuminant as a whole can occur, which are perceptible by an observer albeit only slightly. This effect is additionally intensified in that these colour sequences required for the calibration occur regularly. Furthermore, there are limits to the shortening of the individual measuring periods, because peaks, which can falsify the test results, may occur if individual light sources are switched on and off again within an excessively short interval.